Support a non-html comment in markup

[msmith-techempower]

Describe the problem

Very often, I will build something out as a PoC first in a single Svelte component before proving out the concept and excising reusable sections to their own components. Often, this looks like starting with the piece I want to keep, and commenting out the remaining to test that the base stuff is working as expected, but if I have any html comments (which is the only way I am aware to comment on markup in Svelte) then IntelliSense will break when I try and comment around those html comments.

<!--
   ... 
  <!-- Actual comment that I want -->

  no end-comment-tag because it detected it in the above comment
  and even if it added it...
--> this is technically invalid html and throws an 'unexpected block closing tag' error

Describe the proposed solution

JSX supports this with simple block like {/* some comment */}, but I understand the mustache semantics are a little different in svelte.

Perhaps another type of decorator like {#comment The comment body} or something would be good? The compiler would strip these out at build time and they would be solely for the developers.

Importance

nice to have

[dummdidumm]

I would be ok with allowing {/* some comment */} (more generally allowing empty expressions like {} which would help language tools to give better suggestions). A special block for this would be way too much for such a niche use case.

[Conduitry]

I just spent way too long digging up #5213. One of the things that was requested in the past (which we said no to) was to be able to comment out specific attributes/props on an element/component. Have you thought about where you'd want to allow {/* ... */} to happen in the file? If that's only going to be allowed in certain places, I bet someone's going to ask why, and we should have a good answer for them.

[dummdidumm]

I would allow it everywhere where {undefined} is allowed and treat it as shorthand for that

[roycrippen4]

Currently {undefined} is valid syntax everywhere in a .svelte file except inside of a style tag and html/component attributes. Does that mean the ability to comment out an attribute is not planned since {undefined} is not allowed within the list of attributes?